"Yes, we do need to make adjustments to Medicare and Social Security and Medicaid — and AARP members realize that — but we need to do so without compromising the health and retirement security of the American people or undermining the values that we all cherish," Rand said.
Obama offered Republicans chained CPI in late December as an option during talks on avoiding the so-called "fiscal cliff," angering liberal Democrats on Capitol Hill.
The change would reduce yearly inflationary increases made to federal programs indexed to the CPI, including Social Security.
Republicans support the move, and Obama has signaled it could be part of a major deficit-reduction deal.
On Tuesday, Rand framed his opposition in terms of seniors' declining economic security, rising healthcare expenses and burdensome credit card debt.
He cited AARP reports that found half of U.S. seniors carry medical debt on their credit cards, and that the share of household income consumed by healthcare costs has risen 51 percent over the last 10 years.
"Unless we are able to reverse the trends that are driving the decline of the middle class, many of today’s middle-class workers will not have a middle-class retirement," Rand said.
AARP is not the only group saying no to chained CPI as Washington girds for another budget battle.
"We remain strongly opposed" to the idea, AFL-CIO government affairs director Bill Samuel recently told Salon, calling the proposal a "very substantial benefit cut" for retirees.
On Tuesday, Rand also also repeated AARP's opposition to raising the eligibility age for Medicare.